1. Field of the Invention
The present invention relates to a device for treating working fluid for electric discharge machining, which is used in a wire electric discharge machine or the like.
2. Description of the Related Art
In wire electric discharge machining for machining a workpiece by applying electric voltage between a wire electrode and the workpiece thereby to generate electric discharge between the two, a part of the workpiece is removed from the workpiece due to the electric discharge phenomenon, and this generates machining waste. Similarly, in the die-sinking electric discharge machining, machining waste is also generated. These waste are in a micro-powder form peculiar to the electric discharge machining, and are generally referred to as sludge. Further, during the electric discharge machining, an electrode is worn to generate a micro-waste of the electrode such as wire waste.
In the wire electric discharge machine, for the purpose of cooling the machining area in a machining tank, a working fluid (water, oil or the like) is ejected from upper and lower guide portions for guiding a wire electrode toward a machining area during the wire electric discharge machining. The ejected working fluid is collected, as polluted fluid containing the above-mentioned sludge and is wire waste, from the machining tank into a polluted fluid tank of a device for treating the working fluid, and is then transferred from the polluted fluid tank to a filter device by means of a filter device pump. The polluted fluid becomes a cleaned fluid by using a filtering member in the filter device to filter the sludge and the wire wastes from the polluted fluid, and the cleaned fluid thus obtained is stored in a cleaned fluid tank. The cleaned fluid stored in the cleaned fluid tank is supplied again to the upper and lower wire guide portions and ejected to the machining area, as working fluid for the wire electric discharge machining. In such a manner, the working fluid circulates from the wire electric discharge machining area, through the polluted fluid tank and the cleaned fluid tank, back to the wire electric discharge machining area, so that it is repeatedly used.
The filtering member in the filter device is made of filter paper of a mesh structure, so that the sludge and the wire waste contained in the polluted fluid are caught by the mesh structure of the filter paper and removed from the polluted fluid when the polluted fluid passes through the filter device. Further, the sludge and wire waste caught by the filter paper, if they are deposited on the filter paper, form a kind of mesh structure, which also may catch the sludge and wire waste. As a result, only the cleaned fluid can pass through the mesh structure of the filter paper itself as well as the mesh structure formed by the sludge and wire waste piled together. Thus, the polluted fluid is taken out, as cleaned fluid, from the filter device and flows into the cleaned fluid tank.
However, when the flow speed of the polluted fluid flowing into the filter device is larger than a predetermined value, the sludge and the wire waste reach the mesh of the filter paper at high speed and enter into the mesh to cause an early clogging. Also, the sludge and wire waste do not form a mesh structure on the mesh structure of the filter paper with suitable gaps maintained therein, but deposits on the mesh surface of the filter paper in a dense state so as to stick to the mesh surface, whereby the working fluid hardly passes through it. If the filter device is in this state, the filtration capacity of the filter device rapidly deteriorates and the filter paper (or the filtering member) reaches the end of its life earlier.
Japanese Unexamined Patent Publication No. 2002-283146 discloses a working fluid treatment device for a wire electric discharge machine, in which a hydraulic pressure sensor for sensing a filtration hydraulic pressure is provided between a filter pump and a filtering member to determine the degree of the clogging of the filter, in order to detect the end of the life of the filtering member.
Japanese Unexamined Patent Publication No. 8-1443 discloses a working fluid treatment device for an electric discharge machine, in which polluted fluid is taken out from a polluted fluid tank by a working fluid circulation pump, cleaned by a filter device, and supplied to a cleaned fluid tank, and in which a flow rate of the cleaned fluid supplied from the cleaned fluid tank to the electric discharge machining area is sensed thereby to control a flow rate of the working fluid returned to the cleaned fluid tank by the working fluid circulation pump so as to be slightly larger than the sensed flow rate of the cleaned fluid supplied to the electric discharge machining area.
Similarly, another type of electric discharge machine is also known in which a flow rate of cleaned fluid supplied to an electric discharge machining area is sensed and a target value is determined to be equal to or slightly larger than the sensed value of the flow rate, so that a discharge rate of a filter pump for delivering the working fluid from a polluted fluid tank to a filter is feedback-controlled to be slightly larger than a flow rate of the working fluid supplied to the electric discharge machining area. Also, Japanese Examined Patent Publication No. 7-41512 proposes a method for controlling the filtration of working fluid in the above case, in which pressure is sensed instead of the flow rate and converted to the flow rate by a pressure/flow rate converting portion.
Further, Japanese Examined Utility Model Publication No. 6-32259 discloses a working fluid circulation device for an electric discharge machine, in which, for the purpose of preventing the heat generation from being caused by a constant operation of a pump for supplying working fluid from a discharged fluid tank (a polluted fluid tank) through a filter or the like to a supply fluid tank (a cleaned fluid tank), a fluid-level sensor is provided for detecting the increase of the fluid level of the discharged fluid tank (the polluted fluid tank) up to a predetermined level, or the decrease of the fluid level of the supply fluid tank (the cleaned fluid tank) down to a predetermined level, so that the pump is actuated when the fluid level of the supply fluid tank decreases down to the predetermined level, and the pump is stopped when a predetermined period passes after the start of the operation of the pump or when the fluid level of the discharged fluid tank increases to the predetermined level.
However, an object of the invention described in Japanese Unexamined Patent Publication No. 2002-283146 is to detect the end of the life of the filtering member to solve the above-described problem in which the filtration capacity of the filter device deteriorates rapidly and the life of the filtering member become shorter when the flow rate of the polluted fluid flowing into the filter device is larger than a predetermined value, but is not a countermeasure for prolonging the life of the filtering member.
In the inventions described in Japanese Unexamined Patent Publication No. 8-1443 and Japanese Examined Patent Publication No. 7-41512, the flow rate of the working fluid to be supplied to the filter device is controlled by controlling the flow rate of the polluted fluid delivered to the filter device so as to be slightly larger than the flow rate of the cleaned fluid supplied to the electric discharge machining area. However, the working fluid in the cleaned fluid tank is used not only as working fluid to be supplied to the electric discharge machining area during the machining operation, but also as supplementary fluid for maintaining the fluid level in the machining tank, as working fluid to be supplied to the working fluid cooling device, as working fluid to be supplied to an ion-exchange resin device for regulating the specific resistance of the working fluid, and furthermore as working fluid in a clean fluid tank for the automatic connection of wires. Accordingly, even if flow rate of the working fluid to be supplied to the electric discharge machining area is sensed and the working fluid is supplied to the filter device at a flow rate equal to or slightly larger than the detected working fluid flow rate to supply the cleaned fluid to the cleaned fluid tank, the working fluid in the cleaned fluid tank can still run short.
Further, in the invention described in Japanese Examined Utility Model Publication No. 6-32259, the fluid level in the supply fluid tank (the cleaned fluid tank) is controlled by actuating the filter device pump to supply the polluted fluid to the filter device when a fluid level sensor detects the decrease of the fluid level in the supply fluid tank (the cleaned fluid tank) down to a predetermined level, and stopping the filter device pump when a predetermined period passes over or the fluid level reaches a predetermined level. Accordingly, it is necessary to frequently repeat the actuation and stop of the filter device pump, which imposes an excessive load on a control board or a magnetic switch for controlling the filter device pump as well as the filter device pump itself.